Printable compositions containing silver nanoparticles, processes for producing electrically conductive coatings using the same, and coatings prepared thereby

ABSTRACT

Printable compositions comprising: (a) 5 to 40 parts by weight of silver nanoparticles having a maximum effective diameter of 150 nm, as determined by laser correlation spectroscopy; (b) 50 to 99.5 parts by weight of water; (c) 0.01 to 15 parts by weight of a dispersing agent; (d) 0.5 to 5 parts by weight of a film former; and (g) 30 to 70 parts by weight of metal particles having a maximum effective diameter of 10 μm, as determined by laser correlation spectroscopy; wherein the printable composition has a viscosity of at least 1 Pa·s; processes for producing electrically conductive coatings using such compositions and electrically conductive coatings prepared thereby.

BACKGROUND OF THE INVENTION

There is a demand in principle for electrically conductive structures onsurfaces of objects with poor surface conductivity. With regard to theconductivity, for example, uses in the integration of electroniccircuits into an electronic component by impressing conductive materialon the surface of the component are desirable. Costly composite problemsof components with separate circuits could thereby be minimized. Inparticular, the printing of surfaces of flexible materials withelectrical strip conductors is particularly interesting. The freedom ofdesign of the whole component with a flexible content should no longerbe influenced by the circuit provided.

The application of copper strip conductors is known. These, however, areapplicable to surfaces only with costly deposition and etchingprocesses. Electrically conductive pastes (e.g., conductive silver)which can be subsequently applied to surfaces and are used forcontacting, are a further development.

There is particular interest in printing polymer materials. During theprinting process by which the surface is made conductive, the surface ofthe substrate should not be heated above the softening point (e.g.,glass transition temperature of a polymer surface) of the surfacematerial. In addition, no solvent that dissolves or partially dissolvesthe surface may be used.

Known processes with which structures can be applied to surfacesinexpensively and with good throughput, are screen printing or offsetprinting processes. These two processes, however, place furtherrequirements on the printing substance used. Thus it is known to theperson skilled in the art that inks or dyes that should be used withthese printing processes place minimum requirements on the viscosity ofthe printing ink. The viscosity must be in the range above 1 Pa·s sothat good printing results can be achieved.

U.S. Pat. No. 5,882,722 and U.S. Pat. No. 6,036,889, the entire contentsof each of which are hereby incorporated herein by reference, describeconductive formulations which contain metal particles, a precursor andan organic solvent and which only form conductive structures at asintering temperature from 200° C. upwards. These known formulationshave a viscosity of approximately 10 Pa·s. The formulation can in factbe used for the printing technologies described (screen printing, offsetprinting), but because of the high sintering temperature required, usefor application to surfaces of polymers is restricted.

International Patent Pub. No. WO2003/038002 and U.S. Pat. App. Pub. No.2005/0078158, the entire contents of each of which are herebyincorporated herein by reference, disclose formulations with silvernanoparticles which are stabilized inter alia with sodium cellulosemethyl carboxylic acid. These documents in fact describe the need forpost-treatment, e.g. by heat, or flocculants, but not the processingtemperature or the conductivity of the microstructures obtained from theformulation. Furthermore, the accurate distribution of the nanoparticlesused and obtained is not disclosed, although the size range is less than100 nm. The content of silver particles of the disclosed formulations isnot more than 1.2 wt. %. The viscosity of the printing formulationtypically necessary for the inkjet process provided is approximately 10mPa·s. The formulation is therefore not really feasible for screen oroffset printing.

European Pat. Pub. No. EP 1586604, the entire contents of which arehereby incorporated herein by reference, discloses a silver paste thatis composed of an epoxy resin, silver flakes and silver nanoparticles.This paste forms a conductive film after printing on or application tothe surface of a base material and subsequent heat treatment.Resistances of less than 5×10⁵ ohm/cm are achieved at sinteringtemperatures above 200° C. This high sintering temperature greatlyrestricts the selection of the printable polymer substrates.

International Patent Pub. No. WO 2008/031015, the entire contents ofwhich are hereby incorporated herein by reference, discloses an aqueousformulation which also contains silver flakes. Conductivities of 0.022ohm/square can be achieved with this formulation at 120° C.

HARIMA offers the product line “NP Series Nano-Paste” which is ananoparticles-based silver conductive ink with low viscosity. HARIMA,however, gives sintering temperatures of 210-230° C.

BRIEF SUMMARY OF THE INVENTION

The invention relates, in general, to inks suitable for the productionof electrically conductive printed images, which are based on nanoscalesilver particles and at least one, preferably polymeric, dispersingagent in an aqueous formulation and a process for the manufacturethereof. Various embodiments of the present invention can provideelectrically conductive structures on surfaces by applying suchformulations via screen, flexographic, engraved or offset printingprocesses. This can be achieved by an aqueous silver-containingformulation which, in addition to silver, contains at least one polymer,by applying to a surface by screen, flexographic, engraved or offsetprinting and ultimate thermal treatment of the printed surface so that aconductivity or reflective surface is produced.

Prior to the present invention, the provision of conductive formulationswhich, using elemental silver, make it possible to produce conductivestructures on, in particular, thermally labile surfaces using offset orscreen printing technology had been unknown. Low temperatures in thecontext of the present invention include, e.g., temperatures which arebelow the glass transition temperature of the polymer surface (e.g.,PVC˜80° C.).

The various embodiments of the present invention providesilver-containing formulations which can be applied to a surface byscreen, flexographic, engraved or offset printing and can be sintered tothe surface by thermal treatment at temperatures of ≦140° C., possiblyless than 100° C., to produce conductive structures.

The invention provides a printable composition for the production ofelectrically conductive coatings based on silver particles dispersed inwater, comprising at least

-   -   a) 5 to 40 parts by weight silver metal particles with an        effective diameter of maximum 150 nm, preferably maximum 100 nm,        particularly preferably 20 to 80 nm, especially preferably 40 to        80 nm, determined by laser correlation spectroscopy, wherein the        silver particles have in particular a bimodal size distribution    -   b) 50 to 99.5 parts by weight water and optionally up to 30        parts by weight solvent,    -   c) 0.01 to 15 parts by weight at least of an in particular        polymer dispersing agent,    -   e) 0 to 5 parts by weight additives, preferably 0.5 to 5 parts        by weight, particularly preferably 1 to 4 parts by weight        additives    -   f) 0 to 5 parts by weight conductive, optionally water-soluble        polymers, preferably 0.5 to 5 parts by weight, particularly        preferably 1 to 4 parts by weight conductive polymers        characterised in that the formulation has    -   d) 0.5 to 5 parts by weight, preferably 1 to 4 parts by weight,        thickener,    -   g) and 30 to 70 parts by weight metal particles with an        effective diameter of maximum 10 μm, in particular 500 nm-10 μm,        preferably silver particles or copper particles which are        sheathed in silver,        and a viscosity of at least 1 Pa·s.

The sum of the parts by weight of the components of the formulation isin particular 100 parts by weight.

One embodiment of the present invention includes printable compositionscomprising: (a) 5 to 40 parts by weight of silver nanoparticles having amaximum effective diameter of 150 nm, as determined by laser correlationspectroscopy; (b) 50 to 99.5 parts by weight of water; (c) 0.01 to 15parts by weight of a dispersing agent; (d) 0.5 to 5 parts by weight of afilm former; and (g) 30 to 70 parts by weight of metal particles havinga maximum effective diameter of 10 μm, as determined by lasercorrelation spectroscopy; wherein the printable composition has aviscosity of at least 1 Pa·s. The effective diameter is the averagediameter as determined by laser correlation spectroscopy (suitablemeasuring instrument e.g. Brookhaven BIC-90 Plus).

Another embodiment of the present invention includes processescomprising: (i) providing a substrate; (ii) printing a compositionaccording to any of the various embodiments of the invention on thesubstrate via one or more of screen printing, flexographic printing,engraved printing, and offset printing; and (iii) heat-treating theprinted composition to form a strip conductor.

Yet another embodiment of the present invention includes substratescomprising an electrically conductive coating prepared by any of theprocesses according to the invention.

The determination of size by laser correlation spectroscopy is knownfrom the literature, and is described, for example, in T. Allen,Particle Size Measurements, vol. 1, Kluver Academic Publishers, 1999,the entire contents of which are incorporated herein by reference.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the singular terms “a” and “the” are synonymous and usedinterchangeably with “one or more” and “at least one,” unless thelanguage and/or context clearly indicates otherwise. Accordingly, forexample, reference to “a dispersing agent” herein or in the appendedclaims can refer to a single dispersing agent or more than onedispersing agent. Additionally, all numerical values, unless otherwisespecifically noted, are understood to be modified by the word “about.”

Dispersing agents suitable for use in the various embodiments of thepresent invention preferably comprise at least one agent selected fromthe group: alkoxylates, alkylol amides, esters, amine oxides, alkylpolyglucosides, alkyl phenols, aryl alkyl phenols, water-solublehomopolymers, water-soluble statistical copolymers, water-soluble blockcopolymers, water-soluble graft polymers, in particular polyvinylalcohols, copolymers of polyvinyl alcohols and polyvinyl acetates,polyvinylpyrrolidone, cellulose, starch, gelatine, gelatine derivatives,amino acid polymers, polylysine, polyaspartic acid, polyacrylates,polyethylene sulfonates, polystyrene sulfonates, polymethacrylates,condensation products of aromatic sulfonic acids with formaldehyde,naphthalene sulfonates, lignin sulfonates, copolymers of acrylicmonomers, polyethylene imines, polyvinyl amines, polyallyl amines,poly(2-vinylpyridines), block copolyethers, block copolyethers withpolystyrene blocks and/or polydiallyl dimethyl ammonium chloride.

The dispersing agent is particularly preferably selected from theseries: polyvinylpyrrolidone, block copolyethers and block copolyetherswith polystyrene blocks. Polyvinylpyrrolidone with a molar mass ofapproximately 8000 amu to 400,000 amu (e.g. PVP K15, apolyvinylpyrrolidone with a molar mass of 10,000 amu from Fluka or PVPK90 (molar mass of approximately 360,000 amu) from Fluka) are especiallypreferably used and particularly preferably also block copolyethers withpolystyrene blocks, with 62 wt. % C₂ polyether, 23 wt. % C₃ polyetherand 15 wt. % polystyrene, based on the dried dispersing agent, with aratio of block lengths C₂ polyether to C₃ polyether of 7:2 units (e.g.Disperbyk 190 from BYK-Chemie, Wesel).

A solution (b) selected from the series: C₁ to C₅ alcohol, in particularC₁ to C₃ alcohol, ethers, in particular dioxalane, glycols, inparticular glycerol, ketones, in particular acetone, is particularlypreferably used.

Suitable film formers (d) can be preferably selected from the series:polydimethyl siloxane, polyacrylate, ammonium salts of polyacrylates,siloxanes, wax combinations, copolymers with pigment-active groups,low-molecular polymers, modified cellulose, in particular hydroxyethylcellulose or methyl cellulose, carbon nanotubes and polyvinyl alcohol,preferably hydroxyethyl cellulose, methyl cellulose and carbonnanotubes. Other preferred film formers (d) are selected from the groupof dispersing agents named above, here particularly preferably e.g. thedispersing agent BYK 356 from BYK-Chemie, Wesel, a polyacrylate and BYK154 from the same company, the ammonium salt of an acrylate copolymer.The film formers (d) can also be used in any combinations; it ispreferable to use a combination of hydroxyethyl cellulose and/or methylcellulose with carbon nanotubes.

Suitable additives (e) can be preferably selected from the series:pigments, defoamers, light stabilisers, optical brighteners, corrosioninhibitors, antioxidants, algicides, plasticisers, thickeners,surface-active substances. The additive is particularly preferably areducing agent, such as e.g. formaldehyde, glycerol, ascorbic acid etc.Formaldehyde is especially preferably used as additive.

Suitable conductive polymers (f) can be preferably selected from theseries: polypyrrol, polyaniline, polythiophene, polyphenylenevinylene,polyparaphenylene, polyethylenedioxythiophene, polyfluorene,polyacetylene, particularly preferably polyethylenedioxythiophene incombination with polystyrene sulfonic acid. A conductive salt ispreferably an “ionic liquid”, in particular salts of the type:tetraalkyl ammonium, pyridinium, imidazolium, tetraalkyl phosphoniumwith fluorinated anions.

A particularly preferred formulation is characterised in that the silverparticles (a) have an effective particle diameter of 10 to 150 nm,preferably 20 to 80 nm, particularly preferably 40 to 80 nm, determinedby laser correlation spectroscopy.

The silver particles (a) are preferably contained in the formulation ata level of 10 to 35 parts by weight, particularly preferably 15 to 30parts by weight. The content of dispersing agent (c) is preferably 0.1to 15 parts by weight, particularly preferably 5 to 10 parts by weight.

It can also be advantageous if the particles used are able in the finalformulation to form tight packings which, even at low concentrations andprocessing temperatures, lead to the desired conductivity of the printedstructure. The requirement of the low concentration thereby has purelyeconomic backgrounds. The lower the level of particles can be maintainedwith the same or similar conductivity, the lower are the material costsof the resulting formulation. A replacement of as large contents byweight of particles as possible by other materials is thereforedesirable.

The invention furthermore provides the use of the composition accordingto the invention for the production of electrically conductive coatings,in particular strip conductors.

The invention also provides a process for the production of stripconductors which is characterised in that the new formulation is printedon a substrate surface using a screen printing, flexographic printing,engraved printing or offset printing method and heat-treated inparticular at a temperature of maximum 140° C., preferably maximum 100°C., to remove water residues and optionally solvents and optionally tosinter silver particles present.

A particularly preferable formulation is characterised in that it usessilver particles of different size. It was surprisingly found that adistribution of this type is advantageous for a formation of conductivestructures even at lower contents of the silver nanoparticles. It mustbe assumed that this is caused by filling the intermeshing volumesproduced between the larger particles with smaller ones. This produceslarger, continuous contact areas in the thermal post-treatment of theink. The resulting formulation consequently achieves, at lower masscontent, the same conductivity of an ink with approximately monodispersedistribution at approximately the same effective diameter, or a higherone at the same content by mass and the same effective diameter.

The invention furthermore provides a substrate, in particulartransparent plastic substrate having an electrically conductive coatingobtainable from a composition according to the invention. A substrate inwhich the electrically conductive coating comprises strip conductorswith a conductivity of minimum 5·10⁵ S/m is preferred.

The above-described requirements are furthermore fulfilled by aformulation which contains silver nanoparticles, silver particles,solvents, film formers, dispersing agents and additives. It preferablycontains small silver nanoparticles which—substantially—contain aneffective diameter of 20 to 80 nm, preferably 40 to 80 nm with a bimodaldistribution in a concentration of 5 to 40 wt. %, preferably 15 to 30wt. %. The formulation can be applied for example to polycarbonate, thendried and heat-treated for several minutes at at least 80° C. Highlyadhesive, electronically conductive structures or, with a surfaceapplication, optically reflecting layers, both with high adhesion topolycarbonate, are then obtained.

The silver sols preferably used in the formulation are produced fromAg₂O by reduction with a reducing agent such as aqueous formaldehydesolution (FA) after previous addition of a dispersing agent. For this,the Ag₂O sols are produced batchwise for example by rapid mixing ofsilver nitrate solution with NaOH by rapid stirring or by using amicromixer according to the as yet unpublished German patent applicationwith file number 10 2006 017 696 in a continuous process. The Ag₂Onanoparticles are then reduced with FA in excess in a batch process andultimately purified by centrifuging or by membrane filtration,preferably by membrane filtration. This mode of production isparticularly advantageous because the quantity of organic auxiliarysubstances bonded to the surface of the nanoparticles can thereby bekept low and furthermore a bimodal size distribution can be obtained. Inparticular, no pre-treatment steps, such as e.g. a prereduction in thepresence of polymers, or other post-treatment steps apart from energyinput, such as e.g. activation of a precursor system, or flocculation,are required.

The invention will now be described in further detail with reference tothe following non-limiting examples.

EXAMPLES Example 1 Production of Nanosilver

A 0.054 molar silver nitrate solution was added to a mixture of a 0.054molar caustic soda and the dispersing agent Disperbyk 190 (manufacturer:BYK Chemie) (1 g/l) in a ratio by volume of 1:1 and stirred for 10minutes. An aqueous 4.6 molar aqueous formaldehyde solution was added tothis reaction mixture with stirring so that the ratio of Ag⁺ to reducingagent is 1:10. This mixture was heated to 60° C., maintained at thistemperature for 30 minutes and then cooled. The particles were separatedfrom the unreacted educts in a first step by means of diafiltration andthe sol was then concentrated, for which a membrane with 30,000 Daltonwas used. A colloid-stable sol with a solid content of 20 wt. % (silverparticles and dispersing agent) was produced. The content of Disperbyk190, according to elementary analysis after the membrane filtration, was6 wt. % based on the silver content. An examination by means of lasercorrelation spectroscopy (Brookhaven BIC-90 Plus) gave an effectiveparticle diameter of 78 nm.

Example 2

1.5 g PVP K40 (SIGMA-ALDRICH) and 1.5 Disperbyk 190 (Altana,Byk-Additives) are dissolved in 15 ml 20% nanosilver sol from example 1.30 g silver powder (Metalor K-1332 P) are then introduced into themixture by means of ultrasonic fingers (G. Heinemann, Ultraschal andLabortechnik) at an amplitude of 30% of the maximum performance. Thepaste is then applied to a polycarbonate film (Makrolon®, BayerMaterialScience AG) by means of screen printing and heat-treated at 130°C. A specific conductivity of 2×10⁶ S/m is achieved.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A printable composition comprising: (a) 5 to 40 parts by weight ofsilver nanoparticles having a maximum effective diameter of 150 nm, asdetermined by laser correlation spectroscopy; (b) 50 to 99.5 parts byweight of water; (c) 0.01 to 15 parts by weight of a dispersing agent;(d) 0.5 to 5 parts by weight of a film former; (f) a conductive polymerpresent in an amount of up to 5 parts by weight, and (g) 30 to 70 partsby weight of metal particles having a maximum effective diameter of 10μm, as determined by laser correlation spectroscopy; wherein theprintable composition has a viscosity of at least 1 Pa·s.
 2. (canceled)3. The printable composition according to claim 1, further comprising(e) an additive present in an amount of up to 5 parts by weight. 4.(canceled)
 5. The printable composition according to claim 1, whereinthe dispersing agent comprises a polymer dispersing agent.
 6. Theprintable composition according to claim 5, wherein the polymerdispersing agent comprises one or more selected from the groupconsisting of block copolyethers, block copolyethers having polystyreneblocks, and mixtures thereof.
 7. The printable composition according toclaim 1, wherein the metal particles comprise one or more selected fromthe group consisting of silver particles, copper particles sheathed insilver, or combinations thereof.
 8. The printable composition accordingto claim 1, wherein the metal particles have an effective diameter of500 nm to 10 μm.
 9. The printable composition according to claim 1,further comprising a solvent present in an amount of up to 30 parts byweight.
 10. The printable composition according to claim 13, wherein thesolvent comprises one or more compounds selected from the groupconsisting of C₁₋₅ alcohols, ethers, glycols, ketones, and mixturesthereof.
 11. The printable composition according to claim 1, wherein thefilm former comprises one or more selected from the group consisting ofpolyacrylates, ammonium salts of polyacrylates, siloxanes, polyethyleneglycols, waxes, modified celluloses, carbon nanotubes, polyvinylalcohols, and combinations thereof.
 12. The printable compositionaccording to claim 1, wherein the film former comprises a mixture of amodified cellulose and carbon nanotubes.
 13. The printable compositionaccording to claim 1, wherein the conductive polymer comprises one ormore selected from the group consisting of polypyrrols, polyanilines,polythiophenes, polyphenylene vinylenes, polyparaphenylenes,polyethylene dioxythiophenes, polyfluorenes, polyacetylenes, andmixtures thereof.
 14. The printable composition according to claim 1,wherein the conductive polymer comprises polyethylene dioxythiophene andpolystyrene sulfonic acid.
 15. The printable composition according toclaim 1, wherein the silver nanoparticles are present in an amount of 10to 35 parts by weight and the dispersing agent is present in an amountof 0.1 to 15 parts by weight.
 16. The printable composition according toclaim 1, wherein the silver nanoparticles are present in an amount of 15to 30 parts by weight and the dispersing agent is present in an amountof 5 to 10 parts by weight.
 17. A process comprising: (i) providing asubstrate; (ii) printing a composition according to claim 1 on thesubstrate via one or more of screen printing, flexographic printing,engraved printing, and offset printing; and (iii) heat-treating theprinted composition to form a strip conductor.
 18. The process accordingto claim 17, wherein the heat-treating is carried out at a maximumtemperature of 140° C.
 19. A substrate comprising an electricallyconductive coating prepared by the process according to claim
 17. 20.The substrate according to claim 19, wherein the electrically conductivecoating has a conductivity of at least 5·10⁴ S/m.
 21. A printablecomposition comprising: (a) 5 to 40 parts by weight of silvernanoparticles having a maximum effective diameter of 150 nm, asdetermined by laser correlation spectroscopy; (b) 50 to 99.5 parts byweight of water; (c) 0.01 to 15 parts by weight of a dispersing agent;(d) 0.5 to 5 parts by weight of a film former; (e) an additive presentin an amount of up to 5 parts by weight and (g) 30 to 70 parts by weightof metal particles having a maximum effective diameter of 10 μm, asdetermined by laser correlation spectroscopy; wherein the printablecomposition has a viscosity of at least 1 Pa·s.
 22. A printablecomposition comprising: (a) 5 to 40 parts by weight of silvernanoparticles having a maximum effective diameter of 150 nm, asdetermined by laser correlation spectroscopy; (b) 50 to 99.5 parts byweight of water; (c) 0.01 to 15 parts by weight of a dispersing agent;(d) 0.5 to 5 parts by weight of a film former; a solvent present in anamount of up to 30 parts by weight and (g) 30 to 70 parts by weight ofmetal particles having a maximum effective diameter of 10 μm, asdetermined by laser correlation spectroscopy; wherein the printablecomposition has a viscosity of at least 1 Pa·s.